Continuous rhythmic breathing movements are essential for respiratory homeostasis. Alveolar ventilation is precisely regulated to maintain arterial oxygen and carbon dioxide tensions and pH within narrow limits. For a given metabolic state the alveolar ventilation required to maintain this homeostasis can be attained with a range of respiratory frequencies. Despite this possible variation in respiratory pattern, most species optimize their respiratory frequency to minimize energy expenditure over a range of physiological and pathological conditions. This regulation depends upon integration of the centrally generated respiratory motor pattern with afferent input from several sensory systems including the slowly adapting pulmonary stretch receptors (SAR). SAR are activated physiologically by lung inflation. Lung inflation during inspiration activates SAR and shortens the inspiratory period; occluding the airway at end-inspiration prevents exhalation, maintains a tonic level of SAR discharge and prolongs expiration. These SAR-mediated effects comprise the Breuer-Hering reflexes. Sits of synaptic transmission within the brainstem pathways producing the Breuer-Hering reflexes are important potential sites for processing of the afferent input and for integration with central respiratory motor pattern generation. The goal of this research is to identify the post-synaptic receptors activated by neurotransmitters at central nervous system synapses mediating the reflex responses to activation of SAR. To identify these transmitters two series of experiments will be performed. First, small (nanoliter) injections of selective receptor agonists and antagonists will be made from multibarrel pipettes into brainstem sites which contain SAR-related synapses. Preliminary work indicates that injection of agonists of the likely endogenous transmitter produce respiratory motor effects similar to physiological activation of SAR afferents by standard lung volume manipulations. Antagonists of the endogenous transmitter block the respiratory response both to physiological activation of SAR afferents and to application of the agonist. Once a potential post-synaptic receptor is identified at a given synapse, the second step will be to: a) determine whether single neurons, within each region, which are identified as receiving SAR-related input respond similarly (excited or inhibited) to SAR afferent activation and agonist application, and b) whether appropriate selective antagonists block both the SAR-related and agonist effects but not the response to application of unrelated excitatory agents. Specific synaptic connections between neurons receiving SAR-related afferent input will be established using unit-unit cross-correlation analysis, spike triggered averaging and cycle-triggered histograms. These studies will provide a mechanistic characterization of a primary reflex regulating the depth of inspiration and rate of breathing.